In the wire rods or the spring steel for wire drawing, wire drawability has been improved by controlling microstructural factors, suppressing segregation, or the like. For example, JP-A-11-199977 proposes that pearlite nodule size, a center segregation level, and a lamellar interval of a pearlite structure are controlled in order to improve wire drawability (particularly, rod drawability) of wire rods. JP-A-2000-239797 proposes that mechanical properties of spring steel are appropriately adjusted to improve rod drawability of the spring steel.
For high alloy formation associated with increase in strength of a spring and the like, suppression of supercooled microstructures is also required for the wire rods. Suppression of the supercooled microstructures can be achieved by manufacturing a wire rod having a large wire diameter. However, the wire rod having the large wire diameter exhibits large work hardening due to heavy wire drawing, and furthermore as initial wire diameter is increased, the wire drawing becomes more difficult. Therefore, a wire rod having a large diameter is required to have higher wire drawability.